At apparently random intervals of a few days or a week or so the router
resets itself.

It is detected by sudden loss of LAN/Internet. If I am the same room as
router and quickly look at it, all lights are out except the power
light which is red. After a few seconds the power light goes green and
the router goes through the normal process until all lights are steady
green (as if one had just switched on). Normal service resumes.

There are no known mains power supply problems. (I have a digital clock
which is sensitive to very short mains interruptions - it does not
trip, also no PCs go down). It is not associated with a LAN connected
PC being switched on.

The power supply is from the original OEM AC ADAPTER Model No AA-181AD.

The router is normally left on continuously.

It is connected to BT Yahoo broadband.

Please can anyone suggest what is wrong?